drm/nouveau/devinit/nv04: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 143119
Addresses-Coverity-ID: 143120
Addresses-Coverity-ID: 143121
Addresses-Coverity-ID: 143122
Addresses-Coverity-ID: 143123
Addresses-Coverity-ID: 143124
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>